Is the Mirena coil right for me? Risks, side effects, and periods

Given the vast array of contraceptive choices now available for women, not to mention the differences between brands, deciding which to go for can often become overwhelming. When it comes to the Mirena coil, a long acting reversible contraceptive device that is inserted into the womb, the information that can be found online is no less confusing. Forums contain conflicting information and experiences — but what are the myths, what are the facts, and which side effects might affect you?

The Mirena coil is active for five years and most health professionals recommend it as an excellent form of contraception. It has also revolutionised how gynaecologists manage heavy menstrual bleeding. It differs from the copper coil as, instead of copper, it slowly releases the hormone levonorgestrel, which is a type of natural progesterone.

Levonorgestrel works in three ways: firstly, it increases the thickness of mucus produced from the cervix. Progestins like levonorgestrel increase the weight of molecules that form the mucus, making it dense and difficult for sperm to pass into the womb to fertilise an egg.1 Secondly, levonorgestrel keeps the lining of the womb thin by causing atrophy, and by suppressing the formation of large dilated blood vessels that are needed for implantation of an egg and subsequent pregnancy.2 As a result, it is less likely an egg will implant, and periods become light or absent as there is hardly any lining to shed.

Finally, the Mirena is known as a ‘foreign body’ as it is not normal for something to be sitting inside the womb for 24 hours a day for five years. Thus, a brief local inflammatory reaction (at a molecular level) occurs inside the womb due to a foreign object residing in the cavity. It is completely safe for the Mirena to sit inside the womb despite our body reacting to it. The advantage is that it adds to the Mirena’s contraceptive efficacy as it creates an adverse environment for pregnancy.

Is the Mirena coil safe to use for everyone?

The Mirena is not safe for women who have active or recurrent pelvic inflammatory disease, any STIs, or who have had infection after childbirth. It cannot be inserted in anyone who is between two days and four weeks postnatally (as there is increased chance of expulsion), or who is currently pregnant.

It is also unsuitable for women who have a known womb abnormality, have undiagnosed abnormal genital bleeding, have had a complicated organ transplant, have an irregular heartbeat, have had a molar pregnancy, cervical cancer, have had their cervix removed, had breast cancer or have a past history of breast cancer, or who have a suppressed immune system. Your doctor will be able to advise you on whether or not the Mirena coil is suitable for you.

Key benefits of the Mirena coil

There are some big advantages to the Mirena coil which make it stand out over other forms of contraception. It is one of the most effective and reliable forms of contraception available, and is even more effective than the permanent method of female sterilisation. This is most likely because the combination of the hormonal effects of the Mirena as well as its presence in the uterus are more effective than the physical obstruction of the fallopian tubes as seen in sterilisation. On top of this, the Mirena coil provides protection against endometrial cancer. Endometrial cancer usually occurs when the cells of the lining of the womb become abnormal, which is more likely to occur when the lining of the womb is thick. As Mirena thins the lining, it can reduce your chances of developing cancer.

The Mirena coil can also significantly reduce the pain associated with periods, endometriosis (when the lining of the womb is found outside the womb), and adenomyosis (when the lining of the womb is found within the muscular wall of the womb, resulting in a bulky uterus). Plus, the Mirena coil is extremely effective at reducing heavy menstrual bleeding (menorrhagia) and can drastically improve the quality of life in those suffering from this.

The Mirena could be a good option for those who are unable to take other forms of contraception such as the combined pill. It is suitable for those who cannot take oestrogen, for people who forget to take the pill, for those looking for long-term contraception, and for those suffering from heavy or painful periods. It does not affect breastfeeding, so can be taken after childbirth, and can also be taken by people who are on medication that reduces the effectiveness of oral contraceptive pills. Finally, it can be used by smokers, or those with high blood pressure.

What are the risks during/after Mirena insertion?

Pain

Fear of pain during insertion is common among women considering getting a Mirena coil. Insertion is uncomfortable, but the level of pain differs for every woman as it is affected by the number of children a woman has had vaginally and a woman’s pain threshold. Difficult or painful insertions have been noted in women that have not had children vaginally as the cervix is tightly closed.3 Women are asked to take oral analgesia such as paracetamol and/or ibuprofen prior to insertion, but you also have the option of local anaesthetic — you just need to ask. Local anaesthetic is inserted into the cervix with a very small needle (the size dentists use for the gum). There may be a sting for a second from the local anaesthetic but most women I have administered it to have not found it painful.

In some women the pain/cramping can continue for a while, and is often the most commonly reported reason as to why women discontinue using the Mirena.4 Pelvic cramping can continue on and off for a few weeks as the womb is ‘irritable’, trying to get used to having something new inside it. This eventually subsides and over-the-counter pain relief is usually effective. However, severe pelvic cramping, which women have described as “crippling”, is usually associated with incorrect placement of the coil. If a Mirena is placed correctly at the top of the uterus, you should not have severe cramping.

Infection

In women that are screened negative for infection prior to insertion, the risk of infection is negligible.5 The overall risk of getting an infection is very low (1.6 in 1000).6 The highest risk period for developing an infection is within the first 20 days of insertion.7 Although Mirena insertion occurs in an aseptic, sterile manner, there is always the risk of bacteria travelling from the vagina and into the womb.

The Mirena itself does not cause STIs, however, it does not protect you from STIs.

Fainting/cervical shock

The cervix is linked to the vagus nerve in your body, the nerve that causes your heart rate to slow down and make you feel faint. Some women have a very sensitive cervix and can faint during Mirena insertion. Recovery is usually rapid but if you feel faint, we stop the procedure. We cannot predict the likelihood of a woman fainting. However, having no history of vaginal delivery can put women at increased risk.

Perforation of the uterus

This risk sounds scary, and is when the Mirena makes a small hole in the wall of the womb. The chances of this happening are 1 in 1000, which is rare.8 Perforation is more likely (six times more) in women who are breastfeeding, due to low circulating levels of oestrogen which causes the uterus to be contracted and smaller.9 In women who have insertion between two days and three months after childbirth, risk of perforation is slightly higher as the uterus is ‘soft’, making it more susceptible to perforation.

Expulsion

Expulsion of the Mirena can occur. The risk is 1 in 20.10 It is more likely to occur within the first three months of insertion when the uterus is ‘irritable’, and more likely to occur during your period as your womb naturally ‘contracts’ during that time. Women that have not had children or who have very heavy or painful periods are at increased risk of expulsion.

Ongoing risks and side effects

Ectopic pregnancy

It is important to remember that the Mirena does a very good job of preventing pregnancy (it is 99.9% effective). However, if you do fall pregnant, there is a slightly increased chance of it being an ectopic pregnancy. It is thought that the progesterone in the Mirena can slow the passage of the egg through the Fallopian tube, making it more likely to implant there.11 The overall risk of an ectopic pregnancy over the five years that a Mirena is in place is 1 in 1000.12 This risk is increased with smoking (which also slows the transport of the egg down the tube), and history of pelvic infection (infection causes scarring of the tubes which prevents the egg reaching the womb).

Hormonal side effects

Small amounts of progesterone from the Mirena are absorbed into your body and can cause side effects such as acne, breast tenderness, and headaches. It is thought that progesterone can alter sebum production (the oily/waxy matter produced by your skin) which can result in acne in some women. Breast tenderness can occur as progesterone tends to stimulate glands in the breast and may cause some fluid retention. The headaches that some women report with progesterone-only contraception are likely to be due to the fact that the body’s hormonal cycle is disrupted when starting contraception. However, oestrogen rather than progesterone is more likely to cause headaches and migraine. These effects do go away with time, but some women are more sensitive to progesterone than others, and so it may take longer.

There is no evidence to suggest the Mirena affects your libido. As libido is affected by many psychosocial factors, it is difficult to collect evidence on the sole effect of Mirena on libido. There is also no evidence to suggest that the Mirena causes weight gain. Studies show no causal link, and again it is very difficult to show direct causation due to the many different reasons someone may gain or lose weight.13

The Mirena does not affect your bone mineral density or your oestrogen levels, and it does not increase your risk of having breast cancer or a heart attack. Evidence suggests that there is very little/no increase in the risk of you having a blood clot, and return of fertility after discontinuation is no different to oral contraceptive methods.14

Periods/irregular bleeding

During the first year of having a Mirena, infrequent bleeding, or having fewer or no periods is more common. However, during the first 3-6 months, women may experience irregular, frequent, heavy, or prolonged bleeding as your body tries to get used to the Mirena. As every woman is different, unfortunately as clinicians we cannot predict if you will have heavy or prolonged bleeding. This irregular bleeding seen with the Mirena is not due to one particular mechanism. It is thought that the progesterone effects outweigh the oestrogen effect on the womb, which is essentially the hormone that stabilises the endometrium (womb lining). As a result, there is fragility of the lining, and changes in blood vessels, as well as non-cyclical shedding of the endometrium, which results in irregular bleeding. Irregular bleeding persists in 20% of women at one year of use.15 Prolonged, frequent bleeding is more common with the copper coil as the Mirena will eventually thin the lining of your womb.

Sudden changes in bleeding pattern or particularly heavy bleeding whilst on the Mirena may signify something else going on, such as infection or expulsion of the coil. Women that have used the Mirena during or after the menopause should have bleeding investigated.

I would persevere with a Mirena for six months to one year, if you choose to use it. If there is no improvement with erratic bleeding, then it just may not be right for you, but don’t worry; there are still plenty of other options that may suit you better.

The Mirena is not unsafe, nor is it ‘toxic’ — remember, it wouldn’t be put on the market if it were. While sometimes it can seem as though there are more risks than benefits, you have to remember that most of the risks have a very low chance of actually occurring. As with any contraception, your doctor will be able to advise you on whether or not it is suitable for you.

Shreya is a London-based doctor specialising in Obstetrics and Gynaecology. Her passion in global and public health, family planning, and reproductive rights led her to pursue a masters in Public Health at Harvard. She is a strong believer that women should have easily accessible, accurate information about their health, to enable them to make decisions about their health that are right for them.